H-2K, H-2D, H-2L, TL (thumus leukemic antigen) and Qa-2 are all closely related members of the H-2 gene family. Each of these genese encodes the 44,000 dalton subunit of a glycoprotein found on the cell surface, in close association with beta-2 microglobulin, and is encoded on chromosome 17. Although all of these proteins are encoded on chromosome 17, the major histocompatibility antigens H-2D, H-2K, and H-2L are encoded within 0.33 centimorgans of one another in the major histocompatibility complex while the T1a and Qa-2 genes are encoded on the closely linked T1a region. The major histocompatibility antigens are highly polymorphic and are responsible for graft rejection among inbred mouse strains while the T1a and Qa-2 genes appear to be much less polymorphic. The structure of these H-2-like genes is now being studied using recombinant DNA techniques. Each H-2-like gene appears to consist of eight coding blocks separated by seven intervening sequences. There are 10-15 H-2-like genes in the mouse genome. The chromosomal location and nucleotide sequences of these cloned H-2-like genes will be determined. These studies may reveal the genetic basis for the apparent polymorphism of the major histocompatibility complex encoded antigens, but relative conservation of the T1a region encoded H-2-like genes. These studies may also provide some information about the function and expression of these cell surface proteins.